How can I print a circular structure in a JSON-like format?

Question

I have a big object I want to convert to JSON and send. However it has circular structure. I want to toss whatever circular references exist and send whatever can be stringified. How do I do that?

Thanks.

var obj = {   a: "foo",   b: obj } 

I want to stringify obj into:

{"a":"foo"} 

Answer 1

In Node.js, you can use util.inspect(object). It automatically replaces circular links with "[Circular]".

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Albeit being built-in (no installation is required), you must import it

import * as util from 'util' // has no default export import { inspect } from 'util' // or directly // or  var util = require('util') 

To use it, simply call

console.log(util.inspect(myObject)) 

Also be aware that you can pass options object to inspect (see link above)

inspect(myObject[, options: {showHidden, depth, colors, showProxy, ...moreOptions}]) 

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Please, read and give kudos to commenters below...

Answer 2

Use JSON.stringify with a custom replacer. For example:

// Demo: Circular reference var circ = {}; circ.circ = circ;  // Note: cache should not be re-used by repeated calls to JSON.stringify. var cache = []; JSON.stringify(circ, (key, value) => {   if (typeof value === 'object' && value !== null) {     // Duplicate reference found, discard key     if (cache.includes(value)) return;      // Store value in our collection     cache.push(value);   }   return value; }); cache = null; // Enable garbage collection 

The replacer in this example is not 100% correct (depending on your definition of "duplicate"). In the following case, a value is discarded:

var a = {b:1} var o = {}; o.one = a; o.two = a; // one and two point to the same object, but two is discarded: JSON.stringify(o, ...); 

But the concept stands: Use a custom replacer, and keep track of the parsed object values.

As a utility function written in es6:

// safely handles circular references JSON.safeStringify = (obj, indent = 2) => {   let cache = [];   const retVal = JSON.stringify(     obj,     (key, value) =>       typeof value === "object" && value !== null         ? cache.includes(value)           ? undefined // Duplicate reference found, discard key           : cache.push(value) && value // Store value in our collection         : value,     indent   );   cache = null;   return retVal; };  // Example: console.log('options', JSON.safeStringify(options))